Prior Art
U.S. Pat. No. 4,121,896 issued Oct. 24, 1978 to Shepherd. The mold disclosed has a flexible lip that permits the mold halves to move closer together during curing to compensate for lens material shrinkage.
U.S. Pat. No. 4,113,224 issued Sept. 12, 1978 to Clark. This patent discloses a plurality of passages connecting the mold cavity with the riser. The structure compensates for shrinkage by keeping the material in the riser fluid longer than that in the mold cavity. Machining is essential to obtaining a finished lens.
Copending application Ser. No. 039,100 filed May 14, 1979 and now U.S. Pat. No. 4,209,289 discloses a mold having continuous mating surfaces with the parting line at the junction of the lens edge and anterior lens surface.
Referring to FIG. 1, the female mold half is formed by a frustronconical support 1 having flange 2 extending from the large end. Concave molding surface 3 has continuous flat mating surface 4 extending radially outwardly from periphery 5 of surface 3. Outwardly tapering wall 6 extends from outer edge 7 of mating surface 4 to the narrow end 8 of frustroconical support 1.
The male mold half is formed by frustroconical side 9 with flange 10 extending outwardly from the large end and convex molding surface 11 extending across the small end of side 9. Recess 12 forms a groove extending around the periphery of surface 11 to define the edge shape of a lens cast in cavity 13. Continuous flat mating surface 14 extends radially outwardly from recess 12 to side 9.
The radii of surfaces 3 and 11 are selected to provide a chosen power and base curve in the lens product well known to those skilled in the contact lens art. The surfaces have a common axis 15 when the mold halves are assembled. Registration of the mold halves is obtained by an interference fit between edge 7, at the juncture of mating surface 4 with wall 6, and edge 16, at the juncture of mating surface 15 and side 9. The amount of interference must be such that the shape of neither recess 12 nor surface 11 is distorted when mating surface 4 is in contact with mating surface 14. Riser 17 is formed by difference in taper angles for side 9 and wall 6 and accommodates an excess of lens material necessary to ensure completely filling cavity 13.